EL2020C
50 MHz Current Feedback Amplifier
b
in a lower 3 dB frequency. Attenuation at high
frequency is limited by a zero in the closed loop
transfer function which results from stray capaci-
tance between the inverting input and ground.
Application Information
Theory of Operation
The EL2020 has a unity gain buffer similar to the
EL2003 from the non-inverting input to the in-
verting input. The error signal of the EL2020 is a
current flowing into (or out of) the inverting in-
put. A very small change in current flowing
through the inverting input will cause a large
change in the output voltage. This current ampli-
Power Supplies
The EL2020 may be operated with single or split
g
power supplies as low as 3V (6V total) to as
high as 18V (36V total). The slew rate degrades
g
significantly for supply voltages less than 5V
(10V total), but the bandwidth only changes 25%
g
for supplies from 3V to 18V. It is not neces-
sary to use equal value split power supplies, i.e.,
g
fication is the transresistance (R
)
of the
is
10 ), the current flowing into the
OL
e
6
g
[
very large (
]
. Since R
EL2020
V
R
* I
OUT
&
OL
INV
OL
b
a
5V and 12V would be excellent for 0V to 1V
inverting input in the steady state (non-slewing)
condition is very small.
video signals. Bypass capacitors from each sup-
ply pin to a ground plane are recommended. The
EL2020 will not oscillate even with minimal by-
passing, however, the supply will ring excessively
with inadequate capacitance. To eliminate supply
ringing and the errors it might cause, a 4.7 mF
tantalum capacitor with short leads is recom-
mended for both supplies. Inadequate supply by-
passing can also result in lower slew rate and
longer settling times.
Therefore we can still use op-amp assumptions as
a first order approximation for circuit analysis,
namely that. . .
&
&
1. The voltage across the inputs
2. The current into the inputs is
0 and
0
Simplified Block Diagram of EL2020
Non-Inverting Amplifier
2020–10
Resistor Value Selection and
Optimization
2020–11
EL2020 Typical Non-Inverting
Amplifier Characteristics
The value of the feedback resistor (and an inter-
nal capacitor) sets the AC dynamics of the
EL2020. A nominal value for the feedback resis-
tor is 1 kX, which is the value used for produc-
tion testing. This value guarantees stability. For
a given gain, the bandwidth may be increased by
decreasing the feedback resistor and, conversely,
the bandwidth will be decreased by increasing
the feedback resistor.
10V
A
R
F
R
G
Bandwidth
Settling Time
V
1%
0.1%
a
a
a
a
1
820X
750X
680X
680X
None
750X
170X
76X
50 MHz
50 MHz
50 MHz
30 MHz
50 ns
50 ns
50 ns
55 ns
90 ns
2
100 ns
200 ns
280 ns
5
10
Reducing the feedback resistor too much will re-
sult in overshoot and ringing, and eventually os-
cillations. Increasing the feedback resistor results
10
ELANTEC [ ELANTEC SEMICONDUCTOR ]
